Make sure plugins are not double counted

In the event that a plugin with the same name was attempted to be
installed more than once, the plugin count would double count, and
result in a negative total plugin count.

This prevents actions from taking place, such as opening a popup plugin,
because it verifies that the plugin count is 0 first.

Change-Id: I5805caaa4448117d6ee86f2e2ce5681ab6b6c324

polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-js-api-interface_test.html

@@ -337,6 +337,13 @@ limitations under the License.
       sandbox.stub(Gerrit, '_pluginInstalled');
       Gerrit.install(p => { plugin = p; }, '0.1',
           'http://test.com/plugins/testplugin/static/test.js');
+
+      // testplugin has already been installed once (in setup).
+      assert.isFalse(Gerrit._pluginInstalled.called);
+
+      // testplugin2 plugin has not yet been installed.
+      Gerrit.install(p => { plugin = p; }, '0.1',
+          'http://test.com/plugins/testplugin2/static/test.js');
       assert.isTrue(Gerrit._pluginInstalled.calledOnce);
     });
 

polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-public-js-api.js

@@ -427,14 +427,18 @@
     const src = opt_src || (document.currentScript &&
          (document.currentScript.src || document.currentScript.baseURI));
     const name = getPluginNameFromUrl(new URL(src));
-    const plugin = plugins[name] || new Plugin(src);
+    const existingPlugin = plugins[name];
+    const plugin = existingPlugin || new Plugin(src);
     try {
       callback(plugin);
       plugins[name] = plugin;
     } catch (e) {
       console.warn(`${name} install failed: ${e.name}: ${e.message}`);
     }
+    // Don't double count plugins that may have an html and js install.
+    if (!existingPlugin) {
       Gerrit._pluginInstalled();
+    }
   };
 
   Gerrit.getLoggedIn = function() {